Method and apparatus for enhancement of fracture fluid clean-up with periodic shock waves

ABSTRACT

The method and apparatus for enhancing fluid removal from a fracture in a geologic formation by applying periodic/cyclic shock waves to the fracture in a formation surrounding a wellbore which has undergone fracturing. In accordance with the invention, the method includes the steps of arranging a device attached to the end of tubing inside the wellbore in the vicinity of said fracture for generating shock waves, providing a liquid via tubing into the device for generating shock waves with the amplitude P a  of shock waves determined by following expression: 
       0.3 MPa≦ P   a ≦1.4 P   p −0.8ρgH,
 
     where P p  is the formation pore pressure, ρ is the formation density, g is a gravity acceleration, H is the depth of said fracture, P a  is the amplitude of the shock wave.

BACKGROUND OF THE INVENTION

The present invention relates to hydrocarbon well stimulation and inparticular to methods and apparatus to mobilize and remove fracturingfluids introduced into the fracture zone and surrounding porous media bymeans of applying periodic shock waves.

BRIEF DESCRIPTION OF PRIOR ART

Fracturing the earth from a wellbore is a known technique for enhancingoil production and recovery from an oil bearing bed. A variety ofmethods have been proposed to create both short and long fractures neara wellbore. However hydraulic fracture treatments oftentimesunderperform. In such cases the so-called Frac and Pack completions showa difference between the designed and-effective fracture length. This isdue to creation of a positive skin effect caused in part by stagnantfluids (for instance polymers) retained in the fracture tip and fracturefaces limiting hydrocarbon production (both in rate and capacity) from agiven well. Numerous technologies have been developed to provide skinremoval and fracture clean-up of such stagnant fluids. One of thesemethods is described and claimed in U.S. Pat. No. 6,069,118 wherein awell stimulation method coupled with methods and compositions to removefluid introduced into a subsurface fracture are presented. In thispatent methods are given to create then exploit chemical potentialgradients at the fracture face to induce fluid flow from the fractureinto the formation thereby increasing effective fracture length andimproving fracture conductivity. In another U.S. Pat. No. 7,723,264methods to increase recovery of treatment fluid following stimulation ofa subterranean formation using cationic surfactant coated particles aredisclosed.

These approaches provide the basis for numerous inventions as disclosedin U.S. Pat. Nos. 5,806,597; 5,875,843; 5,960,880; 5,964,289; and6,439,309. Presently, the primary method for removal of stagnant fluidsis a breaker fluid which is pumped into the fracture to lower viscosityof the stagnant fluids so they are more easily removed from the fractureduring flowback. The main disadvantage of all above noted methods is theproblem of delivering a breaker fluid deep enough into the fracture toprovide the effective breaker action on stagnant fluids under theexisting pressure gradient. One approach to resolve this problem is theuse of vibrations or shock waves to increase the mobility of breakerfluid in the fracture thereby enhancing the process of fractureclean-up. One method for increasing fluid mobility is disclosed in U.S.Pat. No. 6,467,542 wherein high frequency vibrations are used fortreatment of the near well zone to remove a skin effect. Thedisadvantage of this method is high attenuation of the high frequencyvibrations in porous medium, limiting the distance over which they areeffective. The use of shock waves for increasing oil mobility/recoveryis disclosed in U.S. Pat. No. 6,899,175 and U.S. Pat. Nos. 125,783 and140,004.

While there have been a variety of methods proposed for cleaning-upfractures around the wellbore, there remains a need for an economicalmethod which provides effective clean-up of fractures.

SUMMARY OF THE INVENTION

Accordingly, a primary object of the present invention is to provide amethod for enhancing fluid removal from a fracture in a geologicformation by applying periodic/cyclic shock waves to the fluids in thefracture and to the surrounding formation which has undergonefracturing. In accordance with the invention, the method includes thesteps of arranging a device attached to the end of tubing inside thewellbore in the vicinity of said fracture for generating shock waves,providing a liquid via tubing into the device for generating shock waveswith the amplitude P_(a) of shock waves determined by followingexpression:

0.3 MPa≦P _(a)≦1.4P _(p)−0.8ρgH,

where P_(p) is the formation pore pressure, ρ is the formation density,g is a gravity acceleration, H is the depth of said fracture, P_(a) isthe amplitude of shock wave;

It is further object of the present invention to provide a method forenhancing of fluid removal from a fracture in a geologic formation inwhich a device for generating shock waves as described in U.S. Pat. No.6,899,175 is installed in the wellbore of at least one offset wellclosest to the at least one well wherein a fracture is created.

It is another object of the present invention to provide an apparatusfor enhancing of fluid removal from a fracture in geologic formationwhich includes a flow line at the surface supplying a liquid frombreaker tank via a pump into the wellbore and the flow line having acheck valve preventing flow of liquid from the wellbore back into theflow line, a tubing string connected to the flow line and extendingdownwardly into the wellbore, an elongated cylinder connected to thebottom of tubing string at the upper end and having an opening towellbore, a plunger movably arranged within an elongated cylinder tomove within the elongated cylinder, the pumping means connected withplunger for moving of plunger within the elongated cylinder andcompressing the liquid contained between check valve inside the flowline and plunger inside the tubing and discharging said liquid into thewellbore via an opening when plunger exits out of the elongated cylinderon every upstroke of pumping means to generate a shock wave, alubricator accommodating a pumping means to prevent the leakage ofliquid from the tubing and flow line at the surface.

It is another object of the present invention to provide an apparatusfor enhancing of fluid removal from a fracture in geologic formation inwhich the pumping means is a wire-line or slick-line.

It is another object of the present invention to provide an apparatusfor enhancing of fluid removal from a fracture in geologic formation inwhich pumping means is a string of sucker rods connected to the pumpingunit installed at the surface.

It is another object of the present invention to provide an apparatusfor enhancing of fluid removal from a fracture in geologic formation inwhich the pumping means upward motion length L_(p) on every upstroke isdetermined by following formulae:

${L_{p} = \frac{4P_{a}{V_{t}\left( {1 - \frac{P_{t} - P_{c}}{\beta \; \phi}} \right)}}{\pi \; \beta \; \phi \; D_{p}}},$

where L_(p) is a length of upstroke of pumping means, P_(a) is therequired amplitude of shock wave, V_(t) is a volume of liquid containedbetween check valve inside the flow line and plunger inside the tubing,π equals 3.1415, β is a bulk modulus of pure water, φ is a coefficientaccounting the difference in compressibility between pure water andliquid contained between check valve inside the flow line and plungerinside the tubing, D_(p) is diameter of plunger P_(t) is a pressure ofliquid inside tubing, P_(c) is a pressure of liquid inside wellbore.

It is another object of the present invention to provide an apparatusfor enhancing of fluid removal from a fracture in geologic formation inwhich the elongated cylinder consists of several cylinders having thesame inside diameter and connected between each other.

It is another object of the present invention to provide an apparatusfor enhancing of fluid removal from a fracture in geologic formation inwhich the plunger has a check valve configured to open for delivering aliquid into tubing above plunger on down stroke of pumping means.

It is further object of the present invention to provide an apparatusfor enhancing of fluid removal from a fracture in geologic formationcomprising: a flow line at the surface supplying a liquid into wellbore,a tubing string connected with flow line and extending downwardly intothe wellbore, an elongated cylinder connected to the bottom of tubingstring at the upper end and having at least one opening into wellbore onthe side surface of the elongated cylinder, a plunger movably arrangedwithin said elongated cylinder to move within said elongated cylinder,said plunger includes a lower portion having a diameter greater thanupper portion of plunger, a spring installed between said lower portionof plunger and the bottom of elongated cylinder and said springundergoes a compression displacement when pressure inside tubing exceedsthe pressure in wellbore causing the lowering of plunger inside theelongated cylinder and the discharging of liquid contained inside tubinginto the wellbore via at least one said opening as far as a top of thelower portion of moving down plunger reaches at least one said openingthereby generating a shock wave, then said spring returns to its initialposition as far as the liquid pressure inside tubing equalizes with thewellbore liquid pressure and the process repeats itself as anauto-oscillation regime with the frequency of auto-oscillations inaccordance with formulae:

${\omega = \sqrt{\frac{Z}{M} - \frac{\lambda^{2}}{4M^{2}}}},$

where ω is a frequency of auto-oscillations, Z is spring constant, M isa weight of plunger and λ is a coefficient of friction between the lowerportion of plunger and the elongated cylinder.

It is another object of the present invention to provide an apparatusfor enhancing of fluid removal from a fracture in geologic formation inwhich said spring has a spring constant Z determined in accordance withthe following formulae:

$Z = \frac{\pi \; {D_{p}\left( {P_{t} - P_{c}} \right)}}{1 - \frac{D_{o}^{2}}{D_{p}^{2}}}$

where Z is spring constant, π equals 3.1415, D_(p) is a diameter of thelower portion of plunger, D_(o) is a diameter of the upper portion ofplunger, P_(t) is a pressure of liquid inside tubing, P_(c) is apressure of liquid inside wellbore.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of a wellbore in which anapparatus and method of the present invention is employed.

FIG. 2 is a cross-sectional side view of the alternative apparatus forpracticing the present invention.

FIG. 2 a is a cross-sectional top view of tubing having at least oneopening and upper part of plunger.

FIG. 3 shows a schematic illustration of alternative method forpracticing the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown a wellbore 1 having perforations 5and fractures 6 with a proppant and stagnant fluid residing in thefracture 6. The stagnant fluid must be degraded which requires thehighly viscous polymers to be broken and the stagnant fluid mobilizedand removed, otherwise the gel inside the fracture 6 can detrimentallyimpede the flow of fluid from the formation into the wellbore 1. Removalof this gel requires a polymer breaking mechanism to be implemented.Liquids called breakers are typically injected into the fracture 6 toaccelerate breaking the polymer. Those chemicals cleave the cross-linkedpolymer molecules into smaller pieces of lower molecular weight. FIG. 1shows a general arrangement of the clean-up apparatus and procedureusing the periodic/cyclic shock waves provided by the device forgenerating such shock waves comprising a flow line 11 at the surfacesupplying a liquid/breaker from breaker tank 13 via pump 12 intowellbore 1, a check valve 10 installed on flow line 11 preventing flowof liquid from the wellbore 1 back into flow line 11, a tubing string 2connected to flow line 11 and extending downwardly into the wellbore 1,an elongated cylinder 3 connected with the bottom of tubing string 2 atthe upper end and having an opening 8 to wellbore 1, a plunger 4 movablyarranged within elongated cylinder 3 to move within elongated cylinder3, the pumping means 7 connected with plunger 4 for moving of plunger 4within elongated cylinder 3 and compressing the liquid contained betweencheck valve 10 inside the flow line 11 and plunger 4 inside the tubing 2and discharging the compressed liquid into the wellbore 1 via opening 8when plunger 4 exits out of elongated cylinder 3 on every upstroke ofpumping means 7 to generate a shock wave 24. A lubricator 9 accommodatesa pumping means 7 to prevent the leakage of the compressed liquid fromtubing 2 and flow line 11 at the surface. The generated shock waves 24have an amplitude P_(a) determined by following expression:

0.3 MPa≦P _(a)≦1.4P _(p)−0.8ρgH,

where P_(p) is the formation pore pressure, ρ is the formation density,g is a gravity acceleration, H is the depth of said fracture 6, P_(a) isthe amplitude of shock wave 24. In particular, for formation porepressure P_(p), the formation density ρ, gravity acceleration g anddepth of formation H accounting for 45 MPa, 2300 kg/m³, 9.81 m/s² and3000 m, correspondingly, the amplitude of generated shock waves 24 hasto not exceed 33.6 MPa. The shock waves 24 propagating through fracture(s) 6 enhance the process of clean-up by breaking the high molecularchains and enhancing the movement of breaker inside the fracture (s) 6and in the formation thereby increasing the effective fracture length.The generation of shock waves 24 described above is based on classichydro-impact phenomenon when compressed liquid contained between checkvalve 10 inside the flow line 11 and plunger 4 inside the tubing 2 isdischarged into wellbore 1 via opening 8 during a fraction of a second.

As pumping means 7 a wire line or a string of sucker rods connected tothe pumping unit installed at the surface could be used. The length ofpumping means upstroke L_(p) to compress the liquid contained betweencheck valve 10 inside the flow line 11 and plunger 4 inside the tubing 2is determined by the following formulae:

${L_{p} = \frac{4P_{a}{V_{t}\left( {1 - \frac{P_{t} - P_{c}}{\beta \; \phi}} \right)}}{\pi \mspace{2mu} \beta \; \phi \; D_{p}}},$

where L_(p) is a length of upstroke of pumping means 7, P_(a) is therequired amplitude of shock wave 24, V_(t) is a volume of liquidcontained between check valve 10 inside the flow line 11 and plunger 4inside the tubing 2, π equals 3.1415, β is a bulk modulus of pure water,φ is a coefficient accounting the difference in compressibility betweenpure water and liquid/breaker contained between check valve 10 insidethe flow line 11 and plunger 4 inside the tubing 2, D_(p) is diameter ofplunger 4, P_(t) is a pressure of liquid inside tubing 2, P_(c) is apressure of liquid inside wellbore 1. In particular, for D_(p)=0.06985m, P_(a)=10 MPa, V_(t)=8.5 m³, β=2.2*10⁹ Pa, φ=0.8, P_(t)=12.5 MPa andP_(c)=12 MPa the length of upstroke L_(p) accounts for 12 m.

The generation of shock waves could be provided without using a pumpingmeans. Referring to FIG. 2 and FIG. 2 a, there is shown a device forgenerating shock waves comprising a flow line 11 at the surfacesupplying a liquid/breaker from breaker tank 13 via a pump 12 into thewellbore 1, a tubing string 2 connected with a flow line 11 andextending downwardly into the wellbore 1, an elongated cylinder 20connected to the bottom of tubing string 2 at its upper end and havingat least one opening 14 into wellbore 1 on the side surface of theelongated cylinder 20, a plunger 21 having a lower portion 19 with adiameter greater than diameter of upper portion 15 of plunger andmovably arranged within the elongated cylinder 20 to move within theelongated cylinder 20, a spring 16 installed between said lower portion19 of plunger and the bottom of the elongated cylinder 20.

The spring 16 undergoes a compression displacement when pressure insidetubing 2 exceeds the pressure in the wellbore 1 causing the lowering ofplunger 21 inside the elongated cylinder 20 and discharging of theliquid contained inside tubing 2 into the wellbore 1 via said at leastone opening 14 as far as the top of the lower portion 19 of downwardmoving plunger 21 reaches at least one opening 14 thereby generating ashock wave, then spring 16 returns to its initial position as far as theliquid pressure inside tubing 2 equalizes with the liquid pressure inwellbore 1 and the process repeats itself as an auto-oscillation regimewith the frequency of auto-oscillations in accordance with formulae:

${\omega = \sqrt{\frac{Z}{M} - \frac{\lambda^{2}}{4M^{2}}}},$

where ω is a frequency of auto-oscillations, Z is spring constant, M isa weight of plunger 21 and λ is a coefficient of friction between thelower portion of plunger 19 and the elongated cylinder 20. Inparticular, for Z=163000 N/m, M=120 kg, and λ=350 kg/sec the frequencyof auto-oscillations ω accounts for 36.8 Hz. The spring constant Z, inturn, is determined in accordance with the following formulae:

${Z = \frac{\pi \; {D_{p}\left( {P_{t} - P_{c}} \right)}}{1 - \frac{D_{o}^{2}}{D_{p}^{2}}}},$

where Z is spring constant, π equals 3.1415, D_(p) is a diameter of thelower portion 19 of plunger 21, D_(o) is a diameter of the upper portion15 of plunger 21, P_(t) is a pressure of liquid inside tubing 2, P_(c)is a pressure of the liquid inside wellbore 1. In particular, forD_(p)=0.06985 m, D_(o)=0.03985 m, P_(t)=12.5 MPa and P_(c)=12 MPa thespring constant accounts for 163000 N/m.

The elongated cylinder 20 has also the opening 18 at its bottom to avoidthe compressing of liquid below plunger 21.

Plunger 21 can be installed and retrieved after clean-up procedure bymeans for instance of a wire-line or a slick-line technique using thecorresponding fishing neck 17 at the top of plunger 21.

Referring to FIG. 3, there is shown a method for enhancing of fluidremoval from a fracture 6 in geologic formation in which a device 22 forgenerating shock waves and described in U.S. Pat. No. 6,899,175 isinstalled in the wellbore of at least one offset well 1 closest to theat least one well 23 wherein the fracture 6 is created.

While in accordance with the provisions of the Patent Statutes thepreferred forms and the embodiments of the invention have beenillustrated and described, it will be apparent to those of ordinaryskill in the art various changes and modifications may be made withoutdeviating from the inventive concepts set forth above.

1. A method of enhancing fluid removal from a fracture in geologicformation comprising the steps of: a) arranging a device for generatingshock waves and said device is attached to the end of tubing inside thewellbore in the vicinity of said fracture; b) providing a liquid viasaid tubing into said device for generating shock waves; c) generating aperiodic shock waves from said device for generating shock waves withthe amplitude P_(a) of shock waves determined by following expression:0.3 MPa≦P _(a)≦1.4P _(p)−0.8ρgH, where P_(p) is the formation porepressure, ρ is the formation density, g is a gravity acceleration, H isthe depth of said fracture, P_(a) is the amplitude of shock wave;
 2. Amethod as defined in claim 1, wherein a device for generating shockwaves and described in U.S. Pat. No. 6,899,175 is installed in thewellbore of at least one offset well closest to at least one wellwherein said fracture is created.
 3. Apparatus for generating periodicshock waves in a wellbore, comprising: a) a flow line at the surfacesupplying a liquid into the wellbore and said flow line has a checkvalve preventing flow of liquid from the wellbore back into said flowline; b) a tubing string connected to said flow line and extendingdownwardly into the wellbore; c) an elongated cylinder connected to thebottom of tubing string at its upper end and having an opening towellbore; d) a plunger movably arranged within said elongated cylinderto move within said elongated cylinder; e) pumping means connected withsaid plunger for moving of said plunger within said elongated cylinderand compressing the liquid contained between said check valve inside theflow line and said plunger inside the tubing and discharging thecompressed liquid into the wellbore via said opening when said plungerexits out of said cylinder on every upstroke of said pumping means togenerate a shock wave; f) a lubricator accommodating a pumping means toprevent the leakage of liquid from tubing and flow line at the surface.4. Apparatus as defined in claim 3, wherein said pumping means is a wireline.
 5. Apparatus as defined in claim 3, wherein said pumping means isa string of sucker rods connected to the pumping unit installed at thesurface.
 6. Apparatus as defined in claim 3, wherein said pumping meansupward motion length L_(p) on every upstroke is determined by followingformulae:${L_{p} = \frac{4P_{a}{V_{t}\left( {1 - \frac{P_{t} - P_{c}}{\beta \; \phi}} \right)}}{\pi \; \beta \; \phi \; D_{p}}},$where L_(p) is a length of upstroke of pumping means, P_(a) is therequired amplitude of shock wave, V_(t) is a volume of liquid containedbetween check valve inside the flow line and plunger inside the tubing,π equals 3.1415, β is a bulk modulus of pure water, φ is a coefficientaccounting the difference in compressibility between pure water andliquid contained between check valve inside the flow line and plungerinside the tubing, D_(p) is diameter of plunger P_(t) is a pressure ofliquid inside tubing, P_(c) is a pressure of liquid inside wellbore. 7.Apparatus as defined in claim 3, wherein said elongated cylinderconsists of several cylinders having the same inside diameter andconnected between each other.
 8. Apparatus as defined in claim 3,wherein said plunger has a check valve configured to open for deliveringa liquid into said tubing above said plunger on down stroke of pumpingmeans.
 9. Apparatus for generating a periodic shock waves in wellbore,comprising: a) a flow line at the surface supplying a liquid into thewellbore; b) a tubing string connected to said flow line extendingdownwardly into the wellbore; c) an elongated cylinder connected to thebottom of tubing string at its upper end and having at least one openinginto wellbore on the side surface of said elongated cylinder; d) aplunger movably arranged within said elongated cylinder to move withinsaid elongated cylinder; e) said plunger includes a lower portion havinga diameter greater than upper portion of plunger; f) a spring installedbetween said lower portion of plunger and the bottom of said elongatedcylinder, said spring undergoes a compression displacement when pressureinside said tubing exceeds the pressure in wellbore causing the loweringof plunger inside said elongated cylinder and the discharging of thecompressed liquid contained inside tubing into the wellbore via said atleast one opening as far as a top of the lower portion of moving downplunger reaches said opening thereby generating of shock wave, then saidspring returns to its initial position as far as the liquid pressureinside said tubing equalizes with the wellbore liquid pressure and theprocess repeats itself as an auto-oscillation regime with the frequencyof auto-oscillations in accordance with formulae:${\omega = \sqrt{\frac{Z}{M} - \frac{\lambda^{2}}{4M^{2\;}}}},$ whereω is a frequency of auto-oscillations, Z is spring constant, M is aweight of plunger and λ is a coefficient of friction between the lowerportion of plunger and the elongated cylinder.
 10. Apparatus as definedin claim 9, wherein said spring has a spring constant Z determined inaccordance with the following formulae:${Z = \frac{\pi \; {D_{p}\left( {P_{t} - P_{c}} \right)}}{1 - \frac{D_{o}^{2}}{D_{p}^{2}}}},$where Z is spring constant, π equals 3.1415, D_(p) is a diameter of thelower portion of plunger, D_(o) is a diameter of the upper portion ofplunger, P_(t) is a pressure of liquid inside tubing, P_(c) is apressure of liquid inside wellbore.